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The name Amphipoda comes, via the New Latinamphipoda, from the Greekrootsἀμφί ("different") and πούς ("foot"), in reference to two kinds of legs that amphipods possess. This contrasts with the related Isopoda, which have a single kind of thoracic leg.[3] Particularly among anglers, amphipods are known as freshwater shrimp, scuds or sideswimmers.[4][5]

The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed, and there is no carapace.[6] The thorax bears eight pairs of uniramousappendages, the first of which are used as accessory mouthparts; the next four pairs are directed forwards, and the last three pairs are directed backwards.[6] Gills are present on the thoracic segments, and there is an open circulatory system with a heart, using haemocyanin to carry oxygen in the haemolymph to the tissues. The uptake and excretion of salts is controlled by special glands on the antennae.[4]

The abdomen is divided into two parts: the pleosome which bears swimming legs; and the urosome, which comprises a telson and three pairs of uropods which do not form a tail fan as they do in animals such as true shrimp.[6]

Amphipods are typically less than 10 millimetres (0.39 in) long, but the largest recorded living amphipods were 28 centimetres (11 in) long, and were photographed at a depth of 5,300 metres (17,400 ft) in the Pacific Ocean.[8] Samples from the Atlantic Ocean with a reconstructed length of 34 centimetres (13 in) have been assigned to the same species, Alicella gigantea.[9] The smallest known amphipods are less than 1 millimetre (0.04 in) long.[10] The size of amphipods is limited by the availability of dissolved oxygen, such that the amphipods in Lake Titicaca at an altitude of 3,800 metres (12,500 ft) can only grow up to 22 millimetres (0.87 in), compared to lengths of 90 millimetres (3.5 in) in Lake Baikal at 455 metres (1,500 ft).[11]

Mature females bear a marsupium, or brood pouch, which holds her eggs while they are fertilised,[4] and until the young are ready to hatch.[6] As a female ages, she produces more eggs in each brood. Mortality is around 25%–50% for the eggs.[4] There are no larval stages; the eggs hatch directly into a juvenile form, and sexual maturity is generally reached after 6 moults.[4] Some species have been known to eat their own exuviae after moulting.[4]

The classification of the Amphipoda is however being rearranged to better reflect their phylogeny, the relationships within the suborder Gammaridea having suffered from the most confusion. A new classification has been developed in the works of Lowry & Myers, where a new large suborder Senticaudata was split off from the Gammaridea in 2013.[14][15] That taxon, which also encompasses the previous Caprellidea, now comprises over half of the known amphipod species.[12]

The classification given below, from the rank of suborder down to superfamily, however still represents the traditional division as given in Martin & Davis (2001),[16] except that superfamilies are recognised here[according to whom?] within the Gammaridea.

Amphipods are found in almost all aquatic environments, from fresh water to water with twice the salinity of sea water.[4] They are almost always an important component of aquatic ecosystems.[20] Most species in the suborder Gammaridea are epibenthic, although they are often collected in plankton samples. Members of the Hyperiidea are all planktonic and marine.[6] Many are symbionts of gelatinous animals, including salps, medusae, siphonophores, colonial radiolarians and ctenophores, and most hyperiids are associated with gelatinous animals during some part of their life cycle.[21] Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non-marine waters. Notably rich endemic amphipod faunas are found in the ancient lakes Baikal and waters of the Caspian Sea basin.[22]

Compared to other crustacean groups, such as the Isopoda, Rhizocephala or Copepoda, relatively few amphipods are parasitic on other animals. The most notable example of parasitic amphipods are the whale lice (family Cyamidae); unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves to baleen whales. They are the only parasitic crustaceans which cannot swim during any part of their life cycle.[25]